[b]Introduction[/b] As more and more cars enter households, vehicle safety has become an increasingly important topic. The Tire Pressure Monitoring System (TPMS) has emerged to address this need. It is the third most important automotive safety electronic product after ABS and airbags. It is mainly used to automatically monitor tire pressure and temperature in real time during vehicle operation and to provide real-time alerts for any abnormalities, serving as a life-saving early warning system for drivers and passengers. Currently, there are two main implementation forms of TPMS: Wheel-Speed ​​Based TPMS (also called "indirect TPMS") and Pressure-Sensor Based TPMS (also called "direct TPMS"). Indirect TPMS determines tire pressure changes by comparing the speed differences between wheels using wheel speed sensors in the automotive ABS system; this method is not widely used now. Direct TPMS uses pressure and temperature sensors in each tire, and then transmits the collected pressure and temperature signals to the main controller in the car's cabin for processing via wired or wireless means. Currently, most TPMS uses wireless transmission of pressure and temperature data. Direct TPMS is now widely used. In this method, once the tire module is installed, the battery continuously operates. Therefore, low power consumption of the tire module, RF receiver sensitivity during high-speed wheel rotation, and noise suppression become key issues in system design. Based on this principle, this paper proposes a new TPMS design method. Experimental results show that the developed system works reliably and can achieve the purpose of safety warning. [b]1 Overall Design and Selection of Main Components[/b] The Tire Pressure Monitoring System (TPMS) operates via radio frequency transceiver and consists of a tire module and a monitor module. Figure 1 shows its system block diagram. [img=246,252]http://image.mcuol.com/News/081006113949980.jpg[/img] [img=307,156]http://image.mcuol.com/News/081006114001851.jpg[/img] 1.1 Tire Module The tire module consists of a sensor, microprocessor, transmitter chip, battery, and antenna. Because this module is embedded inside the tire, its ultra-small size and power-saving design are the most critical issues. 1.1.1 Sensor The sensor selected is Motorola's MPXY8020A pressure/temperature sensor. It is a surface micromechanical capacitive microelectromechanical system (MSMS) pressure sensor. Its features include: dedicated TPMS pressure and temperature sensor, CMOS process, low power consumption, 3 Operating voltage V, integrated low-frequency oscillator with MCU wake-up function, 8-bit digital output; all functions are integrated on a single chip, reducing power consumption and making it suitable for demanding battery-powered systems; pressure measurement range is -40℃ to +125℃, temperature measurement range is 0 to 637 kPa; it has 4 operating modes: standby/reset, pressure measurement, temperature measurement, and data output. Users can select the corresponding mode by setting the S0 and S1 pins, as listed in Table 1. Table 1 shows that the MPXY8020A requires different operating circuits in different operating modes, thus achieving the goal of reducing power consumption. 1.1.2 Microprocessor Microprocessor selection. Microchip's PIC16F636. Its main features include: ① High-performance RISC technology. Only 35 instructions need to be learned, which greatly facilitates program writing, debugging, and modification, and makes it easy to simulate SPI serial ports and open-drain pins in software. ② Extremely low power consumption. The operating current is approximately 100μA at a 1 MHz clock frequency, while the typical operating current in sleep mode is only 1 nA. ③ Wide operating temperature range. The automotive-grade temperature range is -40℃ to 125℃. ④ Complete confidentiality. The PIC uses a security fuse to protect the code. Once the user burns the code and the fuse blows, no one else can read it again unless the fuse is restored. Currently, the PIC uses a deep fuse embedding process, making fuse restoration extremely difficult. ⑤ Built-in watchdog timer. Provides the system with a self-reset function under harsh environments, improving the reliability of program operation. 1.1.3 Transmitter Chip The transmitter chip selected is the Maxim Integrated MAX1479. Its features include: a miniature 3 mm × 3 mm 16-pin QFN package, 3 V operating voltage, automotive-grade temperature range (-40℃ to +125℃), fast-start oscillator (200μs), built-in phase-locked loop (PLL) and high-efficiency power amplifier, support for ASK, OOK, and FSK modulation modes, ultra-low power consumption (standby current is only 0.2 nA at room temperature), adjustable FSK offset, and programmable clock output. 1.2 The monitor module mainly consists of a receiver chip, a microprocessor, an LCD display, and buttons. 1.2.1 Receiver Chip The receiver chip selected is Motorola's MC33594 (Remeo2), a monolithic integrated RF receiver. Its features include: LQFP24 package, fast wake-up (1 ms), built-in 660kHz intermediate frequency bandpass filter, complete voltage-controlled oscillator (VCO), image-eliminating mixer, automatic Manchester encoding/decoding (FSK mode), Manchester encoding clock regeneration circuit, SPI interface, and can be used to design 433.92 MHz OOK/FSK receiver circuits. 1.2.2 The microprocessor used in the microprocessor monitor module is the Motorola MC68HC908GZ16 (GZ16 for short), a 48-pin microcontroller. It is an 8-bit microcontroller from Freescale, based on the 68HC08 architecture, with comprehensive resources, a small size, and suitable for the functional requirements of the monitor module and the automotive operating environment. Its main resources include: one CAN module, one SPI module, one ESCI module, two dual-channel 16-bit timer interface modules, eight 10-bit A/D channels, one basic clock module, 37 general purpose input/output pins, and an 8-bit keyboard wake-up port. This controller uses PLL phase-locked loop technology and can generate a maximum bus frequency of 8MHz. 1.2.3 LOD Display: The LCD display selected is the Samsung LG192641 dot-matrix LCD. It features: 192×64 dot matrix, large viewable area (dimensionals 113.0 mm × 71.0 mm × 9.5 mm, viewable area 97.0 mm × 48.0 mm), built-in LCD control driver, and single 5-bit... V power supply/dual power supply optional, wide operating temperature range (-20℃～+70℃), uses LED backlight and EL backlight optional, good display effect under strong light. [b]2 Hardware Circuit Design[/b] 2.1 Tire Module Circuit Figure 2 shows the schematic diagram of the tire module circuit. The module is installed on the tire valve core and is powered by a 3V lithium battery. The crystal oscillator frequency of the RF chip is 13.56 MHz, the transmission mode is FSK, and the RF frequency is 433.92 MHz. The PIC16F636 uses an internal crystal oscillator, which has strong anti-interference ability. Manchester encoding is used to improve the reliability of data transmission. The formula for calculating the crystal oscillator frequency is [img=516,438]http://image.mcuol.com/News/081006114003622.jpg[/img] 2.2 The circuit diagram of the monitor module, shown in Figure 3, illustrates the circuit principle of the central receiving and processing module. The data manager supports communication between the MC33594 and the GZ16 controller, allowing input of tire pressure thresholds via keyboard. The pressure and temperature values ​​of each tire are displayed more intuitively on the LCD screen. When a tire inflation pressure is abnormal, a buzzer and LED trigger an audible and visual alarm, and the corresponding tire image on the LCD screen flashes as a warning. [img=520,461]http://image.mcuol.com/News/081006114013623.jpg[/img] [b]3 Software Design[/b] The tire module is a system extremely sensitive to power consumption. It is powered by a battery with limited size and weight, and battery and tire module replacements are inconvenient. Therefore, optimizing the tire module's program algorithm to reduce system power consumption is a key design challenge. The monitor module is powered by a car battery, so power consumption is not a primary concern. Its main software design task is to achieve accurate data processing, intuitive display, and abnormal alarms. 3.1 Communication Protocol To achieve one-way wireless data communication between the tire module and the monitor module, a set of communication protocols that both parties must adhere to must be established. 3.1.1 Data Carrier Waveform In this design, the TPMS signal uses Manchester encoding and FSK modulation. The frequency changes corresponding to its "1" and "0" bits are shown in Figure 4 (fdev is the frequency offset value). [img=401,185]http://image.mcuol.com/News/081006114026174.jpg[/img] 3.1.2 The tire module sends data to the monitor module in the form of data frames. The receiving end MC33594 specifies that, when using FSK modulation, the data frame consists of: a 4-bit preamble code, an 8-bit ID, another 4-bit preamble code (MC33594 specifies a 4-bit preamble code before the header), a 4-bit header, user data, and a 2-bit end-of-time (EOM) code. The preamble code is specified as 4 consecutive Manchester-coded "1"s or "0"s used to restore the synchronization clock; the ID and header values ​​are configurable and are pre-written into the MC33594's configuration register by the MCU (the default ID in this design is sixteen). The data frame uses hexadecimal FB8F6 (with a header of "0110"). The header indicates the start of the user data, which follows immediately without any delay. The EOM consists of two consecutive "1"s or "0"s in non-return-to-zero (NRZ) code. The 20-bit code string preceding the user data is defined by the RF receiver chip and is called the "preamble". The preamble for this design is hexadecimal FB8F6. Data frame transmission must end with the EOM; simply terminating the RF signal is not sufficient. Given that tire pressure and temperature values ​​may remain relatively constant for extended periods, transmitting temperature and pressure values ​​is less necessary in such cases. Therefore, this design employs a data transmission scheme combining long and short frames. The specific frame format is as follows: [img=499,113]http://image.mcuol.com/News/081006114028385.jpg[/img] 3.2 Tire Module Programming In the main program design of the tire module, fully utilizing the STOP mode of the PIC16F636's low-power mode is key to the low-power algorithm design. After the PIC16F636 is powered on and initialized, it enters standby mode (i.e., STOP mode). After power-on reset, the sensor is first set to work in STANDBY mode, and then the MAX1479 enters STOP mode. In this mode, the OUTPUT pin outputs a falling edge every 3 seconds, triggering the external interrupt of the PIC16F636, thereby waking up the PIC16F636, causing it to leave the STOP state and enter the interrupt service routine. Data acquisition and transmission control processing are performed in the interrupt service routine. If the acquired value is a new maximum or minimum value (within the transmission cycle), it is stored in RAM; otherwise, the counter is incremented by 1 and the system returns to stop mode. After 10 consecutive wake-ups (30 The module sends its status to the receiver. The module analyzes the difference between the stored maximum and minimum tire pressure values. If this difference exceeds the maximum difference (Δmax) stored in the ROM, module 111' enters fast transmission mode, sending 255 data frames every 800-900 ms. The MAX1479 uses Manchester encoding to transmit radio frequency data. After transmission is complete, external interrupts are allowed again, allowing the sensor to enter STANDBY mode, and the PIC16F636 and MAX1479 simultaneously enter STOP mode to reduce power consumption and extend battery life. The PIC16F636 operates in internal crystal oscillator mode, which can increase its anti-interference capability. The sensor's RST signal resets the PIC16F636 every 52 minutes to further improve the system's reliability. The main program flow of the tire module is shown in Figure 5. [img=336,302]http://image.mcuol.com/News/081006114031406.jpg[/img] 3.3 The main functions of the monitor module program are: monitor module initialization; control of the RF receiving chip; further data processing of the received tire status information (including data display, abnormal status alarms, etc.); and parameter setting of the human-machine interface. The monitor module mainly consists of three parts: main program design, data receiving subroutine design, and human-machine interface program design. The main program flow of the monitor module is shown in Figure 6. The time base module (TBM) inside the GZ16 can generate periodic interrupts. The microprocessor confirms the data received from each tire module, verifying whether data sent by the tire module has been received each time the TBM interrupts. If received, the alarm flag is cleared. When no data is received from a certain tire module for a long time, the data reception timeout flag will be set, thereby triggering the alarm program to remind the driver that the host cannot receive information from that tire module normally. [img=317,327]http://image.mcuol.com/News/081006114035597.jpg[/img] [b]Conclusion[/b] This paper proposes a novel direct TPMS solution and, based on practical development, introduces the system's working principle; it also presents the specific hardware and software design. Vehicle-mounted testing demonstrates that the system exhibits low power consumption, high reliability, good stability, and low cost, making it highly valuable for application. Product manufacturing is currently being planned.